1. Field of the Invention
The present disclosure relates to an image processing apparatus, image processing method, and non-transitory storage medium.
2. Description of the Related Art
Recently, X-ray imaging apparatuses have actively performed tomosynthesis to obtain a desired tomogram from projected images obtained by imaging a subject by irradiating the subject with X-rays from different angles while moving an X-ray tube. This method can obtain a tomogram in a short imaging time without requiring any large-scale apparatuses such as a CT apparatus. For this reason, this technique is high in patient throughput, and has attracted a great deal of attention as a low-exposure imaging technique.
In tomosynthesis, an X-ray imaging apparatus translates (or fixes) an X-ray detector while changing the X-ray irradiation angle in accordance with the characteristics of the apparatus and a necessary tomogram, thereby obtaining a plurality of X-ray images by imaging a subject at different projection angles. The apparatus then reconstructs these X-ray images to generate a tomogram.
In the field of CT, a reconstruction technique using filtered back projection is known as a technique of obtaining mathematically accurate tomograms. As a three-dimensional reconstruction technique using a cone beam, in particular, the Feldkamp method is known as disclosed in non-patent literature 1 (practical cone beam algorithm, L. A. Feldkamp, L. C. Davis, and J. W. Kress, J Opt Soc Am (1984)). This method can directly generate a tomogram by using projected images obtained while making a cone beam face an X-ray detector and rotating the X-ray detector around a subject.
If tomosynthesis reconstruction can be performed by using filtered back projection used in CT in this manner, it is possible to obtain high-contrast tomograms with less blur. However, the positional relationship (geometric arrangement) between an X-ray source and an X-ray detector in tomosynthesis differs from that in CT, and hence it is difficult to directly apply the image reconstruction algorithm used in CT to tomosynthesis. According to patent literature 1 (U.S. Pat. No. 6,256,370), as shown in FIG. 7, a virtual CT detector 7002 corresponding to a detector in cone beam CT imaging is set. This literature describes a method of reconstructing the images obtained by a detector 7001 for tomosynthesis using X-rays from an X-ray source 7000 using a CT reconstruction algorithm such as the above Feldkamp method after temporarily obtaining virtual projection data expected to be obtained by the virtual CT detector 7002.
As shown in FIG. 7, however, when the pixels obtained by the detector 7001, which are uniformly arranged in tomosynthesis, are geometrically transformed into the arrangement of the virtual CT detector 7002, the pixels are nonuniformly arranged. For this reason, pixel values are generated by interpolating the pixel values at the respective points with neighboring pixel values.
However, this interpolating operation corresponds to a spatial low-pass filter, and hence high-frequency information is lost at the time of this geometric transformation. As a result, the tomogram obtained by reconstruction using a cone beam CT algorithm decreases in spatial resolution.
In addition, the technique disclosed in patent literature 1 requires a memory space to geometrically transform a cone-beam CT image and hold the resultant image. Furthermore, the extra processes, namely geometric transformation and interpolation, prolong the processing time required for reconstruction in tomosynthesis whose merit lies in short processing time.